Highly conserved threonine residues were noted near the C-terminus of the external surface glycoproteins of HIV-1, SIV, and influenza A virus; this threonine residue was shown to be the efficient target of O-glycosylation on all three viruses. In all three cases, this O-glycosylated threonine was essential for the infectivity of the virus. We will define the functional role of C-terminal threonine glycosylation for HIV-1 and we will develop assays amenable to high throughput screening for the development of antiviral drugs. We will delineate protein-peptide and peptide-peptide interactions that are dependent on the O-glycosylated threonine of gp120. We will also examine whether there are rare examples of naturally-occurring HIV-1 sequences that are functional without an O-glycosylated threonine at this location.